1. Field of the invention
The present invention relates to a projection unit mounted in a copying machine which is used for projecting an image of a light permeable original such as a slidefilm, liquid crystal panel, electro-luminance (EL) panel or the like, onto a prescribed exposure point on a photosensitive material.
2. Description of the prior art
In recent years, there have been developed copying machines for copying an image of a document placed on a document table which are also capable of copying an image of a light permeable original such as a slidefilm, liquid crystal panel, EL panel or the like. Such copying machines are mounted with a projection unit which is used for projecting the image of the light permeable original onto a prescribed exposure point on a photosensitive material. In such copying machines, for copying the image of the document, an exposure optical system is used to project the image of the document onto the photosensitive material for slit exposure, and for copying the image of the light permeable original, the projection unit is used to project the original image onto the photosensitive material for slit exposure. By projecting the image of the original onto the same position on the photosensitive material as when projecting the image of the document, a copy image of the original can be produced by making use of the existing units of the copying machine in after-exposure processes.
When a slidefilm is used as an original for slit exposure by the projection unit, the slidefilm is scanned with light from a light source. The slidefilm usually consists of a small rectangular film measuring about 1 to 2 cm on one side and a mount for sandwiching the periphery of the film across the thickness thereof. The mount is supported on a slide holder which is moved at a given speed with respect to the light source and the photosensitive material. To move the slide holder at a given speed with respect to the light source, the slide holder is coupled to a driving source such as a motor via a transmission mechanism.
Illumination of the slidefilm mounted on the slide holder is usually performed in a light-shielded box in order to avoid effects of external light. To mount the slidefilm on the slide holder disposed inside the box, the box is provided with an opening through which the slidefilm is inserted for mounting on the slide holder. If the slide holder is not positioned near the opening through which the slidefilm is inserted, the slidefilm may not be securely held in the slide holder, but may drop into the box. It would not be easy to remove the slidefilm once it dropped into the box. It is therefore desirable that the slide holder is positioned near the opening so as to ensure firm holding of the slidefilm in the slide holder.
However, when the slide holder is coupled to a driving source via a transmission mechanism, the transmission mechanism and the driving source must also be moved to move the slide holder from the scanning position where the slidefilm is illuminated and scanned to the position near the opening through which the slidefilm is inserted. Moving the slide holder by hand will require a lot of force because the transmission mechanism and the driving source are also moved together with the slide holder. Furthermore, the construction of the slide holder itself must be sturdy enough to withstand the great force applied to it, which will lead to an increase in the size of the projection unit as well as in costs.
Usually, the slidefilm comprises the film which is covered on its periphery by the mount having a greater thickness than that of the film. The thickness of the mount does not always come to the same, but slightly varies from one slidefilm to another. With the slidefilm mount held in the slide holder, light is projected onto the film, and the light transmitted through the film is projected onto the exposure point on the photosensitive material. The slide holder is retained at a prescribed distance from the exposure point on the photosensitive material when the light is projected onto the slidefilm. Therefore, any variation in the thickness of the mount held in the slide holder will lead to a variation in the distance between the photosensitive material and the film of the slidefilm held in the slide holder disposed at a designated position. As a result, the light transmitted through the film may not be accurately focused onto the exposure point on the photosensitive material.
In the projection unit, the light transmitted through the slidefilm is focused onto the exposure point on the photosensitive material by means of a projection lens. It may be possible to make the focal length of the projection lens variable so that the light transmitted through the film of the slidefilm can be accurately focused onto the exposure point on the photosensitive material. However, a projection lens of a variable focal length is not desirable because it requires a complicated construction. Also, even if the focal length of the projection lens is made variable, it will not be an easy task to adjust the focal length of the projection lens by checking the focusing state of the light transmitted through the slidefilm on the exposure point.
Since the film of the slidefilm is small in size, i.e., usually 1 to 2 cm on one side, the slidefilm image is enlarged for copying. In a copying machine capable of enlarged copying of the document image, the position of the lens in the optical system that scans the document image and the moving speed of the photosensitive material are varied to accomplish copying of the document image enlarged in a specified magnification ratio. With the projection unit installed in such a copying machine, the slidefilm image is enlarged for copying when the moving speed of the photosensitive material is set at a faster speed than the moving speed of the slidefilm with respect to the light source. Thus, with the copying machine capable of enlarged copying of the document image, the enlarged copy image of the small slidefilm can be easily produced.
The film of the slidefilm is usually of a rectangular shape having different lengths between the two pairs of opposite sides thereof. Accordingly, the slidefilm is supported in the slide holder with either the lengthwise or widthwise direction of the film aligned with the moving direction of the photosensitive material, and the magnification ratio is set on the basis of the film length that is aligned with the moving direction of the photosensitive material. Therefore, an accurately enlarged copy image cannot be obtained unless the film of the slidefilm is correctly supported in a prescribed state in the slide holder.